Prepare marinade - In a measuring cup or small bowl, whisk together marinade ingredients and pour half of marinade in a shallow bowl or container with chicken breasts, and set aside remaining portion of marinade for later.
Marinate chicken - Allow chicken to marinate for 30 minutes or up to 24 hours in refrigerator.
Wash and prepare vegetables - While chicken is marinating, wash and prepare vegetables, spinach, basil, olives, and cheese. Place stuffing ingredients on a bowl or plate to have ready when working with raw chicken and set aside sliced vegetables to add to skillet later.
Preheat oven to 350°F.
Prepare a clean, working space for cutting and stuffing chicken breasts. You can place wax paper or parchment paper on countertop or a cutting board to protect surface from raw chicken.
Remove fillets - If the fillets/tenderloins are still attached to chicken breast, cut them off and place them in marinade.
Slice pockets - Slice lengthwise into chicken breast, but not all the way through, creating a "pocket" in chicken breast. (See photos in post)
Cover with marinade - Spoon some marinade into pocket, making sure that all of the chicken is covered in marinade.
Fill the pockets with spinach, basil, cheese, and olives.
Insert toothpicks along the edges of stuffed breasts to help hold filling inside while cooking.
Heat oil in skillet - Heat 2 teaspoons of vegetable oil (or high smoke point oil of your choice) at medium-high heat in 10" oven-proof skillet, such as a cast-iron skillet. (If doubling the recipe, use a 12" skillet.)
Sear chicken in skillet - Add stuffed chicken breasts (and fillets if you have them) and sear on each side for 2-4 minutes, or until golden brown.
Return to oven - Transfer skillet to preheated oven (350°F) and cook for about 10 minutes, turning over after five minutes.
Remove fillets from oven - Once fillets are cooked and have an internal temperature of 165°F, remove the skillet from oven. (The smaller fillets are perfect for kids or reserve the cooked fillets for lunch the next day.)
Add vegetables - Set aside fillets and add raw vegetables to skillet.
Drizzle vegetables and breasts with the reserved marinade/dressing. (Note - do not use the old marinade that you had used with the chicken. Use dressing that has not touched raw chicken.)
Return to oven - Return skillet to the oven to finish cooking, for about 10 - 15 more minutes.
Continue roasting in the oven until the chicken is 165°F and the juices run clear.
Allow chicken to rest - Remove toothpicks and cover lightly with aluminum foil to allow the chicken to rest for 3-5 minutes before slicing.
If you don't have an instant-read thermometer, do yourself a huge favor and buy one. Using a thermometer is critical to know when your meat is safely cooked and keeps you from slicing open your chicken or meat to test for doneness. When you cut open chicken or meat before allowing it to rest, the juices run out and your chicken or meat will be tougher and dryer.Ensure that you do not cross-contaminate any of your ingredients with the raw chicken by separating the amount of ingredients you will need for the stuffing. Discard any unused portions that have been in contact with the raw chicken or your hands while working with the raw chicken.This recipe serves 2 adults generously with two leftover fillets OR the meal can be divided between 3 people with smaller portions.The smaller fillets are perfect for kids - scrumptious chicken tenders they can gobble up while you keep the cheese and olive stuffed breasts for yourself. Recipe can be doubled if cooking for more people. Use a larger skillet (12") if doubling.